Add "clean dead bookmarks" features

Make it easier to find and delete “dead” bookmarks:
use-case/problem:
* you create a bookmark
* you delete the thing the bookmark pointed to (eg “inbox” bookmark, because you moved your inbox since creating the bookmark)
* you [Ctrl+O] to get to your ‘inbox’ which now points to two places: 1. a bookmark leading no-where and 2. a document leading to your new inbox location.
* You mistakenly select the bookmark and end up at the “No document is open” place.
* problem is - you can’t easily find the “dead” bookmark to delete via right-click menu because you have too many bookmarks. You can’t ‘un-bookmark’ the “No document is open” page which the bookmark now directs you to.
* possible solutions:
1. at a “find empty bookmarks” tool somewhere in the burger menu or some such which produces a tidy list you can delete or manage in some way.
2. (and/or) let the “No document is open” page give you the option to delete the favorite you just used.

I run into his problem too sometimes. We could also try accessing the bookmarks in the background, and if the document/item id is no longer valid, make the icon look “broken” so you know which ones to delete.